Two types of White Cell optical time-delay units for antenna beam forming are disclosed. One type of White Cell is a Merged Dual Flipped (MDF) White Cell that contains switched optical delay lines. This component is a composite of two White Cell optical cavities that share portions of a common reflector. Light is directed into either one or the other White Cell cavity according to the angular tilts of an array of optical mirrors located on the common reflector. The other type of White Cell is a Wavelength Tapped Delay (WTD) White Cell that contains optical delay lines of fixed length. These delay lines are accessed by tapping into or out of the single White Cell optical cavity through a set of Fabry-Perot (FP) transmission filters. Different FP filters transmit light of different wavelengths. Thus, the use of different wavelengths of light provides access to different delay line lengths.
The MDF and WTD White Cells can be cascaded together. In this cascade, one type of White Cell can define the antenna beam for one axis (e.g., azimuth) and the other type of White Cell can define the antenna beam for the other axis (e.g., elevation). Alternatively, the two types of White Cell in the cascade can define coarse and fine beam positions. Either White Cell type can be placed closer to the antenna aperture in this cascade, but the order of placement can affect the preferred orientation of the WTD White Cell.
The disclosed White Cell technology may be used with steerable antennas such as phased arrays. It is especially useful for wide band or large antenna systems, for which beam squint would be a problem, for both military and commercial applications.
The White Cells disclosed herein and their cascaded combination achieve true-time-delay beam forming for phased array antennas. The true-time-delay approach and use of optical delay lines permit squint-free beam forming for signals with very large instantaneous band widths and for multi-band signals of very different frequencies. The White Cells are compact and use the same physical space for a large number of optical delay and switching paths.
The MDF White Cell is compatible with long sequences of delay lines having lengths that are a binary progression (lengths that are multiples of two greater than each other). In contrast, prior White Cells for beam forming direct the light successively between at most three different possible segment lengths. Thus, the disclosed MDF White Cell requires a smaller number of optical switches to select a particular time delay.
The WTD White Cell is a free-space optical implementation of a RF Rotman lens. Most prior optical Rotman lenses have been constructed from optical-fiber delay elements. The optical delay paths of the WTD White Cell share the same physical space. Thus, the WTD White Cell can be more compact than prior optical Rotman lenses. Also, the WTD White Cell can use optical wavelength demultiplexing and multiplexing to split and combine signals, respectively. Prior Rotman lenses would require optical-phase sensitive combiners or very long delay paths that exceed the optical coherence length.
Since the WTD White Cell makes use of the optical wavelength to select the lens port (or combine the optical signals) and the MDF White Cell is independent of the optical wavelength, they can be cascaded together. The cascade can achieve more antenna beam angles or positions than either White Cell alone and can be used with larger antennas having more array elements.
A conventional White Cell is an optical cavity comprising three reflective surfaces. It was first described by J. U. White in a publication in Journal of Optical Society of America, vol. 32, pp. 285–288 (1942). The use of White Cells to achieve optical time delays suitable for antenna beamforming is described by Anderson and Collins in U.S. Pat. Nos. 6,388,815 and 6,266,176 as well as in their publications in Applied Optics, vol. 36, no. 32, pp. 8493–8503 (1997) and in Conference Proceedings of 1998 IEEE LEOS, Annual Meeting, pp. 273–274 (1998). These prior art White Cells utilize switched delay lines that require a large number of optical switches to select a given delay path, since those paths are composed of segments having a small number (typically two or three) of different lengths. The approach described in the LEOS publication makes use of binary-length delay segments instead. The publication states that those segments can be constructed from glass blocks (a glass channel with reflective walls is presumed), lens trains (re-imaging the light is presumed) or fibers (to confine or guide the light). The glass blocks and lens trains should work, but are physically large and cumbersome. How the fibers would be connected to the White Cell is not discussed in the LEOS publication. A seemingly straightforward approach might be to have fibers of different lengths and to connect the ends of those fibers to the “delay plane”. But such an approach would not work since light exiting a fiber would be imaged back onto the fiber rather than being directed to the array of mirrors (the DMDs). The present disclosure teaches how to make connections to optical-fiber delay lines in a different and non-obvious way, through additional optical waveguide connectors.
With all of these prior art White Cells, each column of optical switches (the spatial light modulators or the DMDs) define a single programmable delay and have a single set of input and output. In contrast, the MDF White Cell of the present invention can define multiple delays and can have multiple inputs and outputs for each column. The result, when combined with the binary length delay lines, is a more efficient use of the White Cell volume.
The WTD White Cell of this disclosure has a single White Cell cavity and taps light into and out of the White Cell at multiple points in a given column. The prior single-cavity White Cells do not have this capability. This tapping allows the single White Cell cavity to be able to generate simultaneously a variety of delay path lengths with each column. In contrast, prior single-cavity White Cells could generate only a single delay path length for each column.
Some optical implementations of RF Rotman lenses are reviewed by R. A. Sparks in a paper presented at the 2000 IEEE International Conference on Phased Array Systems and Technology (see the conference proceedings, pages 357–360). Sparks and other researchers also have constructed and demonstrated various optical Rotman lenses. These prior works do not associate different optical wavelength with different lens ports. U.S. Pat. No. 6,348,890 by Ronald R. Stephens, which patent is owned by the assignee of this application, describes the use of multiple optical wavelengths with an optical Rotman lens implemented with optical fiber delays. Proper choice of these wavelengths permits efficient combining of delayed signals with photodetectors (as described by Stephens in U.S. Pat. No. 6,452,546, which patent is owned by the assignee of this application).
The WTD White Cell disclosed herein makes use of free-space optical delays that are confined within a White Cell cavity instead of the optical fibers. Thus, the disclosed WTD White Cell implementation can be more compact than other implementations. A prior art Rotman lens that uses free-space optical delay paths is described by Curtis in SPIE Proceedings, vol. 2481, pp. 104–115 (1985). That prior lens performs the signal combining in the optical domain and is sensitive to the optical phase differences in the various paths to a given lens port. In contrast, the WTD White Cell disclosed herein makes use of optical-heterodyne signals combining at the photodetector. Thus, the signals are combined in the RF domain and that combining process is not sensitive to the optical phases.
The disclosed MDF White Cell is different from the White Cell described in the LEOS publication in that it has multiple input/output ports for each column of reflector switches. Also, it makes use of optical waveguides (preferably formed on a substrate) whose ends are tilted with respect to the endface of the substrate. This tilt causes the light to enter and exit the waveguides at an angle. The tilted waveguide entrances/exits thus appear like a standard reflective surface since incoming light that is angled with respect to the endface results in outgoing light that is at the corresponding opposite angle. The tilted waveguides are used for both accessing the delay lines of the MDF White Cell and accessing the input/output fibers. The tilted waveguides make it possible to have multiple input/output ports (and multiple switched delay lines) in each column. So far as the inventor is aware, such tilted waveguides have not been used or associated with White Cells or with optical methods for antenna beamforming.
The WTD White Cell is an optical implementation of the Rotman lens and is based on free-space optical delay paths confined in an optical cavity. Prior optical Rotman lenses do not use both free-space optical delay and optical cavity confinement. The WTD White Cell also uses taps in the cavity to obtain different delay path lengths. Prior optical Rotman lenses use optical fibers cut to different lengths, instead. Tapped optical delay lines have been used to produce time delays for antenna beam forming. Such an approach is described by Li and Chen in IEEE Photonics Technology Letters, vol. 9, no. 1, pp. 100–103 (1997). This prior approach taps light out from every upper-side reflection of each delay line path and does not make use of different optical wavelengths. In contrast, the approach of the present disclosure taps light out from only one of the upper-side reflections of a given delay-line path for a given wavelength. Different wavelengths tap the light from different upper-side reflection points.